Your writing is not letters on the page - it's what happens as you write. The writing is an artifact, a fossil record. If you focus more on the writing experience, and less on the outcome, you'll get more done. You'll be more "disciplined."
(It's not really about discipline, but about loving what you do.)
Do this: Honor your writing time. Make your desk, chair, writing-place inviting and comfy and spoiling and all that good stuff. Supply chocolate.
Set a timer for fifteen or twenty minutes with the intention of keeping your pen or keyboard keys moving all the while.
When it's over, don't count pages or words. Just consider yourself successful!
Oh, and keep the writing as a byproduct. (Shhh. Don't let it see you.) Don't even read it. Be nonchalant as you slip it into a folder of practice writing.